I dont find it hard and i always say it when needed 

I think there are times where I replace "thank you" for "sorry. But I think it's pretty simple to do it, especially when you feel the other's party sincerity.

I used to find it hard to say thank you because i was shy! 
Not anymore though i enjoy saying thank you now, its good to treat people with respect!

I find it too easy to say 'Thank you'. That's not a good thing. People tend to climb all over me if I'm always appreciative and agreeable to stuff.

Not at all. If someone's doing something (anything!) to help you, they deserve a thanks.

Though I've come across people who don't seem to think this...

No i don't find it difficult, but sometimes I forget. :/ Otherwise I definitely will. 
However, I find it especially frustrating when someone asks for a 'thank you'.
In this case, I honestly don't feel as thankful (although I should be!! >.<)
It contradicts the idea that a genuine act of kindness is given without expectations.

not at all difficult! I say " Thank you " every now and then. A simple " Thank you " can change someone's day tbh :') and i definitely appreciate it when you do something & someone says " Thank You " x 

I say thank you all the time, it's the polite and right thing to do.

I say thank you.